Is this a new oven or a new problem with an old oven? In a gas oven all the heat comes from the bottom. It will balance out a little more as it becomes preheated. I would first have it checked new or old to see if it is working correctly. If a new oven, and it is working correctly, you might try insulating the bottom of your dish by either an aluminum cookie sheet directly underneath or on the shelf below. You might also make sure your oven is totally preheated when you put your food in. I would avoid pyrex or any type of glass as this will hold the heat. It is always a good idea to get an independent oven thermometer and check the temp in different areas of the oven.